Traditionally, sintering aid addition to ceramic materials which undergo a liquid phase densification is performed by simply milling a solid sintering aid powder with the ceramic material in the presence of an alcoholic solvent, normally isopropanol. The milling serves both to comminute the ceramic particles to a smaller and more uniform particle size and to distribute particles of the sintering aid somewhat uniformly among the ceramic particles. The result is a simple mixture of different particles which inherently means that some portions of the mixture will be richer in sintering aid particles while other portions will be poorer in them. This variation in sintering aid concentration is believed to be a cause for failure of some ceramic parts, particularly at elevated temperatures. In addition, due to the inherent non-uniformity the total amount of sintering aid needed to produce dense bodies from the ceramics is quite high. This is generally undesirable, especially for high temperature performance, because it can result in the presence of an excessive amount of glassy phase. There is need for ceramic material/sintering aid compositions which contain a lower than conventional amount of sintering aid, but have the sintering aid distributed more uniformly so that the resultant densified bodies exhibit desirable high temperature properties. This is particularly true for silicon nitride compositions.
Furthermore, the comminution of ceramic materials such as silicon nitride and silicon carbide being performed in conventional alcoholic solvents is both expensive and hazardous. The ability to perform the comminution in water without causing degradation of the ceramic material or generation of explosive levels of gases would be highly desirable. The present invention evolved from seeking a solution to each of these problems and a recognition that they are interrelated.
Previous attempts to resolve these problems has been directed at solving each individually, rather than simultaneously. U.S. Pat. No. 3,830,652 (Gazza) claims a densifiable silicon nitride composition containing an yttrium compound wherein the weight of yttrium in the yttrium compound is about 1.0 to 3.5 weight percent of the weight of the silicon nitride. This simple approach of merely using lower than normal amounts of yttria sintering aid would be wonderful, except that, as shown by comparative example below, it has not been found to produce silicon nitride bodies having sufficient high temperature properties to be commercially useful. As a result, currently produced silicon nitride bodies generally contain a minimum of about 4% by weight yttria to have sufficient high temperature properties to be commercially useful.
Another attempt at reducing the amount of sintering aid is disclosed in Japanese Publn. 62-265,171 which teaches sintering a silicon nitride material with a sintering aid which has been prepared by spraying aqueous metal salts in a solvent into a plasma flame. A reduced sintering aid usage is alleged, but only a simple mixture of silicon nitride particles and sintering aid oxide particles is produced.
A further method of adding a sintering aid is shown in Japanese Publn. 62-187,170 in which a mixture of silicon nitride particles and silicon carbide whiskers is impregnated with a solvent-based solution of yttrium hydroxide or alkoxide and then the impregnated hydroxide or alkoxide is transformed to an oxide. The process is similar to that of Shaw et al., discussed below, and produces a substantial amount of yttrium oxide particles intimately blended with silicon nitride particles and silicon carbide whiskers as well as a small amount of the nitride and carbide having a partial yttria coating.
A still further attempt is disclosed in Japanese Publn. 62-158,166 in which silicon nitride powder is mixed with a solution containing a metal nitrate, the solvent simply evaporated, and the mixture dried and baked to convert the nitrate to an oxide. The solvent is stated to be water or a 1-4 carbon alcohol. The process does not yield particles of silicon nitride uniformly coated with a sintering aid oxide because after addition of the metal nitrate to the silicon nitride particles the powder must be dried and pulverized before baking to convert to an oxide. The intermediate pulverization step means that many fresh silicon nitride surfaces are produced which can not possibly have any sintering aid thereon. Although water is suggested as a possible solvent there is no disclosure in the abstract of any step being taken to prevent or even deter the degradation of the silicon nitride that must occur by reaction with the water. Also no steps are taken to prevent the formation of individual particles of sintering aid oxide which are less efficient than a coating in promoting densification.
Japanese Publn. 61-281,069 mixes silicon carbide and silicon nitride powders in a solution containing a metal alkoxide and then calcines the mixture until the alkoxide is hydrolized. The result is predominantly a mixture of particles of silicon carbide, silicon nitride, and metallic oxide, with a small amount of metal oxide particles formed on the surfaces of the carbide and nitride powders.
Japanese Publn. 61-251,578 forms an alcoholic solution of a metal alkoxide and uses that solution in place of a metal oxide powder as a sintering aid source for silicon nitride. The result is a mere blend of silicon nitride and metal oxide particles with a slight amount of the metal oxide possibly adhering to some of the silicon nitride particles, but not completely coating them.
T. M. Shaw and B. A. Pethica in "Preparation and Sintering of Homogeneous Silicon Nitride Green Compacts", 69 Journal of the American Ceramic Society 88-93 (1986) teach a means to obtain a more uniform distribution of sintering aid than by conventional milling. Specifically, they teach the precipitation of yttrium, magnesium, and/or aluminum hydroxides by adding solutions of the corresponding metal nitrates to a suspension of silicon nitride powder in water which also contains tetraethylammonium hydroxide in sufficient quantity to cause precipitation of the metal hydoxide from the dispersion medium. The precipitate and the suspended silicon nitride particles, some of which may be partially coated with the precipitate, are jointly flocculated to produce a well-mixed powder of silicon nitride and densification aiding metal hydroxides. Such mixtures were found to sinter to higher final densities under the same sintering conditions than mixtures of the same chemical composition formed by conventional joint milling of constituents initially introduced into the suspension in powder form.
The general preparation of solid materials in finely divided form is a frequent practical need. Generally the process of comminuting solids is most effectively accomplished while the solids are suspended in a fluid that lubricates the flow of the particles as they interact with the milling balls, knives, or the like that are used to accomplish the actual division of the relatively large solid particles into smaller ones. The division of solid particles into smaller ones necessarily produces new solid surfaces and therefore promotes chemical reaction between the new surfaces and constituents of the environment in which the surfaces are formed. At times, these reactions are deleterious to the properties desired in the fine solid material being prepared and/or generate explosive levels of gases, so that preventing such reactions would be advantageous.
One of the well-known examples of a deleterious reaction between a suspension medium and a solid material being comminuted is silicon nitride and water since the silicon nitride readily reacts with water leaving a substantial coating of silica on the silicon nitride surface. It is known that in the presence of water silicon nitride undergoes the reaction: EQU Si.sub.3 N.sub.4 +6H.sub.2 O=3 SiO.sub.2 +4 NH.sub.3
And in the presence of water, silicon carbide undergoes the reaction: EQU SiC+2 H.sub.2 O=SiO.sub.2 +CH.sub.4
which again results in the presence of substantial amounts of silica on the particle surfaces. The silica in each case can reduce the usefulness of the particles particularly at high temperatures. Also the water can generate dangerous amounts of ammonia or methane. Based upon these problems, milling of silicon nitride and silicon carbide in water, while having been attempted, has previously generally been avoided. Also, milling in water has been found to cause extreme morphological changes in the powder by forming extensive hard agglomerates which have impeded densification and adversely affected the mechanical properties of the resultant dense body. As a result, conventional practice in the art is to mill silicon nitride or silicon carbide in an organic solvent, such as isopropanol, to produce the very fine powders used for making shaped articles having desirable physical properties, particularly at high temperatures. Alcohol is more expensive and more hazardous, due to its potential for fires and explosions, than water. Also it is becoming more difficult to dispose of due to environmental considerations. It would be clearly more advantageous to use water as the milling mediium.
Although the presence of small amounts, i.e. less than about 2%, of silica in silicon nitride is normally not detrimental, larger amounts are generally undesirable in the intergranular phase of silicon nitride articles intended for service at high temperatures. In addition, some densifying aid oxide is needed to permit adequate densification under known practical temperatures and pressures. The most preferred densification aid at present is yttria, with magnesia an alternative for lower temperature uses, and mixtures of these constituents with each other, with a small amount of silica, or with other metal oxides being known in the art. Conventionally, oxide sintering aids have been milled together with the silicon nitride or silicon carbide powders in alcohol. While this has produced adequate results for many applications, it is generally believed that a more uniform distribution of sintering aid than can now be achieved by conventional milling in alcohol is desirable to permit a reduction in the total amount of sintering aid needed.